On “principles of operation,” motor drive circuits are cousins of computer power supplies. DC motors, like switching power supplies, are driven by a stream of pulses and their controllers are pulse width modulators. The speed of the motor then becomes a function of phase and frequency of the pulses it receives. As with computer power supplies, motor drive system designers are looking for higher energy transfer efficiency, enabling smaller size motors and drivers, shorter latency and lower costs. Add to the list of care-abouts: lower noise.
The design of motor drive systems ― from the thimble-sized electric tooth brushes to the elephant-sized industrial conveyors ― is undergoing a transformation. Even under Industry 4.0 guidelines, we’re looking for high-horsepower motors that are smaller, more energy efficient, and quieter. This represents a challenge for component suppliers who must balance microcontroller-based “Intelligence” with “brute force” power switches and amplifiers. As a trend, most of the semiconductor makers are targeting the mid-range and fractional horsepower motors, 5-to-50 W (consuming 2-to-5 A in use). This is the operating point for a wide variety of motors and actuators including those for printers, vending machines, and copiers ― even hotel door locks.
Counting “actuators,” as well as miniature open loop rotors, almost 10 billion motors are delivered each year (certainly, millions per month). Electric motors consume easily 50% of the world’s electricity, many of the contributors to this article believe. The number is difficult to pin down, since there is no standard or universal motor drive circuit or form factor to count. Except for tiny motors, controllers and drive trains have been implemented with discrete semiconductor devices.
There are primarily three motor types designers need to understand, reminds Jeff DeAngelis, managing director for Industrial & Healthcare products at Maxim Integrated. These include direct current (DC) brushed motors, brushless DC motors (BLDCs) and AC induction motors. With brushed DC motors, the motor is a coil wound around a rotating armature, driven by a series of inductive pulses to the armature winding within the permanent magnetic field of the stator. The speed and position of the rotor is a function of the stator pulses. Brushed DC motors can function open loop, or closed loop for servos. Stepper motors, operating open loop (in, say, vending machines or office equipment), are a subset of BLDCs). By counting pulses as the armature advances, the stepper can control of its rotational speed and position.
The third type, AC induction motors, are the simplest to design width, since they operate from the available power lines. They offer the lowest cost per watt, in constant-speed applications (like ceiling fans or home appliances), but they can be noisy, and make motor speed and position a challenge to control. The AC motors powering household fans and power run at a constant speed. They are relatively efficient in fixed-speed applications; increasing their efficiency is the goal of modern control circuits. https://www.maximintegrated.com/en/app-notes/index.mvp/id/4697
Maxim Integrated’s motor control ICs target mid-range industrial motors, says DeAngelis. Of the evolving motor applicates, about 40-45% of new designs use DC brush motors, 30% involve DC Brushless Motors, and some 15% use stepper motors with open loop. There is a migration, says DeAngelis, among DC motor systems, from brush motors to brushless motors. The brushless motors are driven harder, to spin the rotor in a shifting magnetic field. But since the rotor does not make mechanical contact with the stator, it generates fewer sparks, and demonstrates higher reliability and lower noise in use.
Semiconductors for Motor Control resist integration
As with computer power supplies, the design of motor drive ICs and modules are governed by the need to shrink their size and increase their efficiency. High integration is fashionable, but not always feasible. Elevated temperatures may require heat sinks for the power stage (switches and gate drivers) which may make component integration difficult and impracticable. Hotel door locks or electric tooth brushes are powered by single-cell lithium batteries; all components can fit on one chip. But reducing the size of a motor drive for a consumer washing machine running on 3-phase 220 Vac (using 650 V IGBTs) does not readily lend itself to single-chip integration.
Until recently, motor control circuits were not highly integrated (except for custom circuits), but rather implemented with discrete gate drivers and transistors and microcontrollers. Because of that it is difficult to size the motor driver IC market. Though the common architecture of the motor drive IC is pulse width modulators, there are many controller implementations that use separate µCs, separate powers stages and separate sensor banks for closed loop control (in applications drawing 5-to-50 W). Single-chip motor controls serve lower-power motors and actuators (2 to 5 W), but higher power controls are typically implemented with discretes.
Though BCD fabrication processes allow power MOSFETs to occupy the same substrate as CMOS logic, the need to dissipate heat favors once again separate power transistors ― MOSFETs, bipolar transistors, and IGBTs ― on aluminum heat sinks. With newer-generation mid-power motors (up to 5 W), component integration is a bit more feasible, though integration is most readily applied to the power stage, effecting a full- or half-bridge driver configuration. H-Bridge drivers include both high-side and low-side FETs; the high-side devices interface with the positive supply rail of the inverter; the low side is in touch with ground.
Consequently, semiconductor content is grouped according to whether the devices serve controller of a sophisticated motor driver, or the power train. For closed-loop servos, engineers will use sensor inputs for position and/or speed sensing. These can be as simple as current-sense resistors or as complex as instrumentation amplifiers with high-resolution A/D converters and/or rotary encoders (See Figure 1).
Figure 1 The most sophisticated motor control modules have three-main components: A microcontroller for measuring motor speed, torque and/or position; a three-phase power stage, and (for closed loop control) an array of sensors. A full- or half-wave bridge will alternate power sourcing for the motor coils.(Source: Maxim Integrated)
Driving brushed DC motors.
On brushed DC motors, current through the rotor coils creates a magnetic field, opposing the magnetic field of the permanent magnets (north and south), and forcing the rotor coil to move. The mechanical commutation will change the direction of the current, and keep the motor spinning. The scraping of the brushes against the rotor, however, generates noise and wear (See Figure 2).
Figure 2: On brushed DC motors, current through the rotor coils creates a magnetic field, opposing the magnetic field of the permanent magnets (north and south). The resulting magnetic forces are applied to the rotor coil making it move. The mechanical commutation will change the direction of the current, and keep the motor spinning (Source: ON Semiconductor)
Be sure to read Part 2 as it takes a look at how to tweak you motor control devices to improve efficiency and how to reduce noise sensitivity, the trending use of new technologies like gallium nitride in motor control circuits.